Device for control of antenna illumination tapers comprising a tapered surface of rf absorption material



Aprifi 11, 1967 J LADER ETAL 3,314,071

DEVICE FOR CONTROL OF ANTENNA ILLUMINATION TAPERS COMPRISING A TAPERED SURFACE OF RF ABSORPTION MATERIAL Filed July 12, 1965 2 Sheets-Sheet 1 Norma/13 9 pan er 60: if; 4 6

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April 11, 1967 LADER ETAL 3,314,071

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3,3 14,971 Fatented Apr. 11, 1&6?

3,314,071 DEVICE FOR CONTROL OF ANTENNA lLLUll il- NATION TAPERS COMPRISING A TAPERED SURFACE OF RF ABSORPTION MATERKAL Leon 3'. Lader, Los Angeles, and Jay B. Winderman,

Pomona, Calif, assignors to General Dynamics Corporation, Pomona, Calif., a corporation of Delaware Filed July 12, 1965, Ser. No. 47Ltl89 Claims. (Cl. 34:5912) This invention relates to devices for shaping the radiation pattern of an antenna, particularly to radar antenna radiation pattern shaping means, and more particularly to devices for controlling illumination taper and sidelobe level of an antenna.

Various means have been devised in the past to eliminate or at least to reduce antenna radiation in a particular direction. The known means have included radiation absorbing devices which are mounted adjacent to the antenna to absorb radiation in a chosen direction. In the known constructions the radiation absorbing material has been fixedly attached to an immovable object adjacent to the antenna such as a radome but has not been satisfactory for use with moving antenna structures such as the antenna structures employed with radar gear because movement of the antenna changes the relative position of the absorbing material with respect to the antenna and thereby changes the direction of maximum radiation absorption relative to the radiation pattern. Other known means have utilized rings mounted at predetermined positions with respect to the periphery of the antenna and function to cancel out or reduce the magnitude of the sidelobe. Other known means have positioned rings or layers of radiation absorbing material on the reflector of the antenna and thus can be moved with movement of the antenna structure. The US. Patent No. 3,101,473 exemplifies such prior known devices.

In general, no real antenna is perfect in terms of optimum sidelobes and beamwidth for a prescribed aperture. Since low sidelobe levels can be achieved only at the expense of increased beamwidth, some compromise must be reached. Furthermore, departures from perfect focusllllg and distortions in the surface of the antenna cause a non-uniform phase distribution, or phase error, over the surface.

Directive radar antennas are designed on the basis of a compromise among a number of factors, chiefly: aperture dimensions, beamwidth, sidelobe characteristics, and cost. A common result of this compromise is that the radar system is forced to live with nominal sidelobe levels which create persistent clutter and ambient noise problems. Further degradation results from minor inaccuracies in fabrication. As an example, an increase in sidelobe level of 10 db, due to small inaccuracies, is common in this type of antenna.

When an antenna has been constructed as a result of the above stated compromise, little can be done to provide an economical cure for departures in phase and amplitude distributions from the desired optimum, except by introducing an additional variable. The present invention provides for appropriately treating portions of the surface of the antenna with an RF absorptive material, whereby the sidelobecharacteristics are controlled at the expense of a small reduction in overall gain.

It would appear preferable to achieve the desired antenna pattern by classical methods using Cos, Dolph, Tchebychev, Taylor, or other standard distribution. However, while control of phase and amplitude distributions by one of these methods is relatively straightforward for very large antennas which warrant great efforts in With the loss variable method of this invention, a theoretical and practical basis for achieving improved patterns is available. Calculations of gain patterns of antennas with and without surface treatment for linear, quadratic, and cubic phase errors have been derived. Experimentally determined effects of the surface treatment upon the gain patterns of a paraboloidal antenna and a two-dimensional slotted array have verified the calculations.

Therefore, it is an object of this invention to provide means for reducing antenna radiation in a chosen direction, such as the sidelobes of the radiation pattern so that the effective radiation is concentrated as desired.

A further object of the invention is to provide a way for controlling illumination taper and sidelobe level of an antenna.

Another object of the invention is to maintain a large ratio of a main antenna radiation beam to a secondary or sidelobe radiation.

Another object of the invention is to provide a means for uniformly reducing antenna radiation in a chosen direction relative to the antenna regardless of the direction in which the antenna is facing.

Another object is to provide an inexpensive, lightweight and compact device capable of absorbing antenna radiation through use of thin film tapered absorbers.

Another object of the invention is to provide a radiation absorbing device for an antenna which reduces sidelobe radiation Without substantially reducing, distorting or displacing the desired radiation pattern.

Another object of the invention is to provide a means to increase the accuracy of radar antennae by maintaining a large ratio between the magnitude of the principal radiation lobe and the sidelobes.

Other objects of the invention, not specifically set forth above, will become readily apparent from the following description and accompanying drawings wherein:

FIG. 1 is a cross-sectional view of an antenna reflector constructed in accordance with the invention;

FIGS. 2 and 3 are graphs illustrating the invention;

FIG. 4 is a plan view illustrating a manner of constructing the tapered RF absorption material of the FIG. 1 antenna; and

FIGS. 5 and 6 are graphs illustrating the invention.

Basically, the invention relates to a method for shaping or altering the radiation patterns of antennae as by a physical change of the antennae configurations. The method essentially involves the application of a tapered ring-like surface layer or provision of equivalent surface treatment to desired portions of a given antenna, the tapered layer preferably comprising an absorptive, dielectric, or magnetically permeable material. The advantage of the method of shaping an antenna beam is that rings of various cross-sectional tapers, according to the desired characteristics, can be applied quickly and easily.

As pointed out above, the present invention is directed to appropriately treating portions of the surface of a directive antenna with absorptive, dielectric, or magnetically permeable material, whereby the radiation pattern can be altered to thereby diminish sidelobe level. Using an RF absorptive material, analytic and experimental investigations involving a parabolic reflector-type antenna and a two-dimensional slotted array have been conducted for verification.

For an antenna with little or no phase error, decreasing the sidelobes is accompanied by a simultaneous broadening of the beamwidth. However, when large phase errors are present, the gain of the first sidelobes of the untreated antenna is comparable to the gain of the main beam, and proper treatment will decrease both beamwidth and sidelobe level.

Quadratic phase error is the predominant result of defo'cusing. For a slotted array, for example, the error can arise from deviations in the position and excitation of the slots from their ideal values. It is assumed that the basic antenna can be treated as a circular aperture with a uniform amplitude distribution and some quadratic phase error. If the radius of the aperture to unity is normalized, the gain function may be determined in equation form. From the solution of that equation, an expression for one-way power can be obtained. Now, consider the application to the antenna surface of a ring of lossy material whose outside diameter equals the diameter of the antenna and whose inside diameter is treated as a variable. The effect of this application is the production of a non-uniform amplitude distribution. Thus, rings of almost any desired cross-section taper can be produced. These linear tapers are thickest at the outer perimeter and may be reasonably well approximated by a parabolic amplitude distribution over the entire surface. The gain function for this case can be interpreted as the superposition of two gain functions. The solution leads to the one-way spatial power distribution or pattern.

These equations are plotted for phase errors of 0, 1r/4, 1r/2, and 31r/4, each curve being normalized for a maximum, or horesight, gain of zero db.

While the phase error in the untreated antenna increases from zero, the beamwidth remains essentially the same, but the levels of the nulls and sidelobes grow rapidly. For small phase errors, tapering the illumination sharply reduces the sidelobes while it increases the beamwidth somewhat. As the phase error grows large, the main beam and first sidelobes of the untreated antenna are nearly indistinguishable and eventually the effective beamwidth is increased appreciably. The present invention now makes it possible, by proper treatment, to reduce both the sidelo'bes and the beamwidth.

To verify the above theory, an experiment was performed using a two-dimensional slotted array and various tapered absorption rings.

The experiment was conducted in an RF darkroom in which the reverberation energy received Was approximately 30 db below the maximum detected signal. A 4 by 5 inch horn connected to an X-band generator constituted the source of radiation. The antenna was a nearly circular slotted array with a mean diameter of approximately 11 inches. It was connected through a crystal detector to a recorder. In order to assure that the far field antenna pattern was obtained, the distance between the source and the array was made greater than D=effective diameter of array \=wavelength of radiation.

Three RF absorption rings were formed from wedgeshaped segments (not shown) of carbon-impregnated sponge material, the rings being of a general configuration as illustrated in FIG. 1 except that they were of a segment type ring construction. Other materials such as a graphite-loaded adhesive could also have been used. All three rings had outside diameters of 11 inches and maximum edge thickness of inch. The width of the rings were 1.0, 1.5, and 2.0 inches, respectively.

The source antenna was fixed and the test antenna was allowed to scan in synchronization with the recorder drive. The pattern of the untreated antenna was taken to provide a reference. The experiment was then repeated with each ring taped to the surface.

The antenna power pattern through the H-plane was plotted on a graph. The pattern for the untreated antenna was non-symmetrical and showed definite evidence of phase errors. The 1.0 inch ring had little effect on the overall shape. However, nearly a 2 db reduction in sidelobe level resulted when the 1.5 inch ring was applied. The 2 inch ring caused a 1 db decrease in one sidelobe and a 1 db increase in another. Thus, an optimum taper exists which will produce a significant reduction in side- 4.- lobe levels. In all cases, the over-all main beam was widened, although the 3 db heamwidth remained essentially constant.

It is thus seen that by appropriately treating portions of the surface of a two-dimensional slotted array, for example, with RF absorption material, it is possible to reduce the level of the sideiobes. For an antenna with a small quadratic phase error, sidelobe reduction will be accompanied by an increase in beamwidth. However, when the phase error is sufficiently large, a simultaneous reduction of sidelobes and be-amwidth can 'be achieved. This method of aperture illumination tapering can be applied readily to any existing slotted array, and it can result in significant improvement where the machining and design tolerances can not be held sufficiently close.

With the above described rings affixed to the surface of a defocused 12 inch paraboloidal antenna with predominantly quadratic phase errors, a decrease of 6 db in the level of the first sidelobes was achieved.

In addition to the above, further theoretical and experimental determinations were made which included a directive antenna consisting of a circular aperture with uniform amplitude distribution and a linear phase error, and a similar antenna with a quadratic error or cubic phase error.

In a paraboloidal antenna, one cause of quadratic phase error is the proper alignment of the feed along the geometrical axis, but not at the correct focal distance. In other types of antennas, the quadratic error may be caused by departures from ideal element dimensions and inter-element spacing.

Cubic phase errors in a paraboloidal antenna 'can be produced by an off-axis feed. This type of error is a natural characteristic of conical scan tracking antennas because the feed is not aligned with the geometric axis.

These experiments were made utilizing a paraboloidal antenna it} provided with a tapered layer 11 of RF absorption material, as shown in FIG. 1, the absorption material 11 being described in detail hereinafter.

Normalized sample calculations of one-way power for quadratic phase errors of zero and 1r/4 radians are plotted in FIGS. 2 and 3, respectively. As the phase error in the untreated antenna increases from zero, the levels of the nulls and sidelobe peaks grow rapidly and change their angular positions with respect to the main beam. Tapering of the illumination significantly reduces the .levels of the first sidelobes while it increases the width of the main beam skirts somewhat.

The Gaussian taper chosen was of the form where k was taken as 4.6 in order that the loss at the outer edge of the aperture would be 20 db greater than the loss at the center. Other values of k could have been selected, especially if a maximum specified reduction in overall gain were an important design consideration. Nevertheless, with the analytic tapers described herein, the reduction of side-lobes relative to the main beam is predictably greater with the Gaussian taper than with the parabolic taper.

In this experiment, the effect of surface treatment on the beam pattern of an optimally focused paraboloidal antenna was investigated. The treatment consisted of a truncated conical ring of tapered RF absorptive material applied to the outer, nearly linear, portion of the paraboloidal reflector, as illustrated in FIG. 1, and constructed as illustrated in PEG. 4.

The truncated conical surface 11 which substantially conforms to the surface of the parabolic antenna reflector 10 near the outer edge thereof was formed from a flat circular piece 12 as shown in FIG. 4 which was out along its radius R as indicated by dash lines 13 and formed into a cone by abutting or overlapping the cut edges. The center material of piece 12 was then removed, the resultant product being a truncated conical surface 11 contoured to the parabolic shape. The flat cincular piece 12 constitutes a substrate of material such as a suitable plastic coated with thin layers of colloidal graphite in such a way as to produce the desired taper in the radial direction as shown in FIG. 1, this being accomplished, for example, by revolving the substrate on a turntable during coating with the colloidal graphite to assure uniformity of taper for all elements of the cone, or ring. As shown in FIG. 4, an uncoated portion 14 of the substrate 12 may be used to facilitate the insertion, retention, and removal of the tapered surface 11. different types of antennas and thus may remain flat or configured to other than conical shapes.

The RF absorptive material could be applied directly to the surface of the antenna 10, but the thin substrate permits the tapers 11 to be interchanged and, hence, permits selection of the desired beam pattern. Also, the absorptive material can be integrated with any weatherproofing or stabilizing compound, such as epoxy, or it can be protected by a separate coating.

This experiment was conducted at X-band, on an outdoor antenna range with a reverberation noise over 36 db below the maximum signal received. A '12 inch paraboloidal antenna with a Cutler feed was used as the test antenna. The source antenna was fixed and the test antenna was allowed to scan in synchronization with a pen recorder. The pattern through the principal plane of the untreated antenna was taken to provide a reference. The experiment then was repeated with each absorptive taper in turn afiixed to the surface of the antenna.

The experimental tapers consisted of truncated conical rings of 0.07 inchMylar, coated with a layer of colloidal graphite as described above. The graphite thickness was tapered so as to produce a monotonically increasing loss from the inner radius to the outer radius. FIG. 5 shows the contours prepared for the experiment. Both linear and inverse Gaussian tapers were created.

The antenna power patterns through the H-plane are shown in FIG. 6. Surface treatment caused a reduction in the level of the first sidelobes of 5.5 db and a corresponding decrease in main beam gain of only db. No perceptible widening of the main beam occurred. The vestigial sidelobes were reduced by one and three db each, and the lower portion of the main beam skirts became narrower. Several of the outer sidelobes also were suppressed. There is no indication that these levels of sidelobe reduction are the maximum achievable.

. The above described experiments have thus shown how the absorptive surfaces made in accordance with this invention can be used to modify the aperture distributions of directive antennas in order to reduce sidelobes. The technique of the invention is not limited to any specific antenna or design described, but can be applied to other types, directly or on a substrate. Furthermore, other absorptive, dielectric, or ferromagnetic material also may be used for controlling sidelobe characteristics. Primarily resistive absorptive material is preferred since reactive component type material may introduce additional phase errors.

The inventive method of aperture tapering can be applied to existing antennas or it can be taken into account during predesign. The variety of tapers achievable greatly exceeds the number which can be created using the classical design techniques. Furthermore, when the absorptive material is applied to a substrate, the radiation pattern of a given antenna can be corrected or made to produce any one of a number of trade-offs by interchanging tapers.

Although specific embodiments of the invention have been illustrated and described, modifications and changes will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications and changes as come within the spirit and scope of the invention.

The surface 11 may be used on- What We claim is:

1. A device for precision control of antenna illumination tapers comprising a tapered surface of radio frequency absorption material adapted to be positioned near the outer edge of an antenna reflector, said surface increasing in thickness from the inner edge to the outer edge thereof.

2. A device for controllingillumination taper and sidelobe level of an antenna comprising a tapered surface adapted to be positioned near the outer edge of an antenna reflector, said tapered surface being constructed of a substrate and a tapered layer of RF absorption material coated on the substrate, said tapered layer being tapered in a radial direction.

3. The device defined in claim 2, wherein said absorption material consists essentially colloidal graphite.

4. The device defined in claim 2, wherein the absonption material includes carbon-impregnated sponge-like material.

5. The device defined in claim 2, wherein the absorption material includes a graphite-loaded adhesive.

6. In combination with a directive antenna, means for diminishing sidelobe radiation levels comprising a tapered layer of RF absorption material positioned near the outer edge of the antenna reflector, said tapered layer having an increasing thickness in the direction of the outer radius thereof.

7. The combination defined in claim 6, wherein said antenna is of the parabolic type.

8. A method of producing sidelobe radiation diminishing means for a parabolic shaped antenna comprising the steps of coating a flat circular substrate with RF absorptive material, applying the coating in such a manner as to produce a desired taper in the radial direction, and forming the coated substrate into a truncated conical configuration adapted to substantially correspond to the surface of a parabolic antenna.

9. The method defined in claim 3, wherein the step of forming the substrate into a truncated conical configuration includes the steps of removing a radially extending section of coated substrate, abutting the edges of the substrate, and removing the undesired uncoated center portion of the substrate.

10. The method defined in claim 8, wherein the step of applying the coating includes revolving the substrate to assure uniformity of taper.

References Cited by the Examiner UNITED STATES PATENTS 2,643,338 6/1953 Brady 3439 14 X 3,078,461 2/1963 Dwyer 34318 3,151,324 9/1964 McMillan 34318 3,156,917 11/1964 Parmeggiani 343-84 X OTHER REFERENCES November 30, 1962, Electronics, Spuncast Plastics Achieve Reflector Precision, by J. W. Dawson.

References Cited by the Applicant UNITED STATES PATENTS 2,460,869 2/ 1949 Braden. 2,492,358 12/ 1949 Clark. 2,512,147 6/1950 Gardner. 2,897,491 7/ 1959 Young. 2,939,140 5/1960 Troost. 3,064,258 11/1962 Hatkin. 3,101,473 8/1963 Fenlon. 3,119,109 1/1964 Miller et a1. 3,176,301 3/1965 Wellons et al.

FOREIGN PATENTS 800,466 8/ 1958 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner. P. L. GENSLER, Assistant Examiner. 

1. A DEVICE FOR PRECISION CONTROL OF ANTENNA ILLUMINATION TAPERS COMPRISING A TAPERED SURFACE OF RADIO FREQUENCY ABSORPTION MATERIAL ADAPTED TO BE POSITIONED NEAR THE OUTER EDGE OF AN ANTENNA REFLECTOR, SAID SURFACE INCREASING IN THICKNESS FROM THE INNER EDGE TO THE OUTER EDGE THEREOF. 